1. Field of the Invention
The present invention relates to a magnetic powder for use in magnetic recording medium, and particularly to a magnetic powder for use in magnetic recording medium that is suitable for constituting the upper recording layer of multi-layer coated-type magnetic recording medium.
2. Background Art
Magnetic recording medium such as tape for home audio-visual equipment and storage tape for data backup have been the subjects of attempts to improve electromagnetic conversion characteristics with the object of increasing the recording density and improving picture quality. With conventional general-use coated-type magnetic recording material, it has been preferable to use magnetic powder with as small of particle size as possible to increase the recording density, and to this end, attempts to reduce the particle sizes are now being done at a rapid pace.
As recited in the various literatures, the typical procedure for preparing metal magnetic powder is to grow iron carbonate or iron hydroxide in water through the use of an aqueous solution of ferrous salts and carbonic acid or an alkaline hydroxide or both. Air or another oxidizing gas is blown through the aqua-dispersion of this precursor to generate iron oxy-hydroxide, and the acicular iron oxy-hydroxide thus obtained is used as the raw material that goes through the dehydration and reduction processes to produce the metal magnetic particles or so-called “metal powder.”
However, it is known that as the particle size becomes smaller, the surface activity of the metal magnetic powder becomes extremely high and if oxygen is present in the handling atmosphere, oxidation will proceed rapidly, leading to marked deterioration in magnetic characteristics.
Improving the stability of this metal powder with respect to oxidation, or namely improving its weatherability, is a problem directly related to the storage stability of the magnetic recording medium itself, so it has been studied energetically from diverse angles. For example, there have been studies focused on the ignition temperature using TG/DTA to increase oxidation resistance by defining these values as disclosed in JP2003-59707A (Patent Document 1), studies focused on changes in different oxidation atmospheres to study high-temperature oxidation stability as disclosed in JP2002-367142A (Patent Document 2), studies focusing on the x-ray diffraction intensity before and after an oxidation reaction to study the chemical composition as disclosed in JP2001-313207A and JP2004-13975A (Patent Documents 3 and 4), studies of the chemical composition of the oxide layer covering the surfaces of the particles as disclosed in JP2001-237115A (Patent Document 5), studies of covering the surface with a substance that gives oxidation resistance as disclosed in JP5-144618A (Patent Document 6), studies of optimizing the thickness of the oxide layer covering the surfaces of the particles as disclosed in JP2003-119503A (Patent Document 7) and the like.
In their studies in regard to reducing the particle size and improving the oxidation resistance of the metal powder, the present inventors found that it is difficult to discuss oxidation stability by merely studying the results of thermogravimetric(TG)/differential thermal analysis(DTA), referred to TG/DTA, and ignition temperatures. Namely, even if the ignition temperature is high, the oxidation resistance may not necessarily be good. In addition, while the thickness of the oxide layer of the particles can be adjusted by varying the oxidation time, if the layer is made thicker, the volume of the magnetic core will become smaller, resulting in problems such as poor magnetic characteristics. Changing the chemical composition of the oxide layer of the particles may require the addition of different types of metal, but the addition of different types of metal may work effectively in improving the oxidation resistance of the magnetic powder, but also lead to problems where the medium characteristics deteriorate when it is made into recording medium, or in certain cases, there may be problems with waste fluid arising at the time of manufacture, so the financial burden may increase since existing disposal facilities may be inadequate, thus requiring new investment.
Accordingly, it is an object of the present invention to obtain a magnetic powder for magnetic recording medium that is both resistant to oxidation and has good magnetic characteristics, and to thus provide magnetic recording medium with improved reliability.